Japanese Patent No. 3331793 shows an engine control system in which an engine speed is computed based on an output of a crank angle sensor. The engine speed is filtered at a processing interval which is synchronized with the engine speed. A variation amount in engine speed, from which an affect of combustion variations in each cylinder removed, is computed based on the filtered engine speed. The variation amount in engine speed is feedback-controlled so as to be in a target range. The variation amount in engine speed is referred to as an engine speed variation, hereinafter.
In the above engine control system, the engine speed variation is computed after the engine speed is filtered at the processing interval which is synchronized with the engine speed. When the processing interval becomes longer than a processing interval of a feedback control which is performed based on the engine speed variation, it is likely that the filtered engine speed for feedback control may not be varied even though an actual engine speed is varied. In this case, it may be erroneously determined that a feedback correction does not work effectively to increase a feedback amount. Thus, an output of the feedback control (feedback amount) becomes unstable.